Demands for lithium ion secondary batteries as batteries for use in consumer electronic equipment such as cellular phones and personal computers and electric vehicles have been rapidly increased, and a large market is prospected. The positive electrode of lithium ion batteries is typically manufactured by applying a positive electrode material prepared from a mixture of a positive electrode active material, an electricity-conducting material and a binder to a single side or both sides of a current collector such as an aluminum foil, followed by drying and pressing.
Examples of the typical conventional positive electrode active material for use in lithium ion secondary batteries include a complex oxide of lithium and a transition metal such as LiCoO2, LiNiO2, and LiMnO2 having a layer structure and LiMn2O4 having a spinel structure. Various studies have been made to improve the characteristics required for a positive electrode active material such as the capacitance density, the charge and discharge cycle durability and the safety. From the viewpoint of achieving high capacity of lithium ion batteries, studies focusing on molybdenum have been recently performed in addition to the typical transition metals such as Co, Ni, and Mn.
According to the description in Japanese Patent Laid Open No. 2010-135325 (Patent Literature 1), cathode active materials of Li2MoO3 doped with various different elements achieve improved electrical characteristics and can be used in various secondary batteries such as lithium batteries. And disclosed are complex oxide compounds of Li2MoO3 with a part of molybdenum substituted with a different element M. Specifically, a positive electrode active material that contains a compound represented by LixMyMozO3 is disclosed, wherein x, y and z satisfy: 0.1≤x≤2.3, 0<y≤0.3, and 0.7≤z≤1.1; and M represents at least one selected from the group consisting of Ga, Ge, Mg, Zn, Cd, K, Na, Ca, Si, Cu, Sn, B, P, Se, Bi, As, Zr, Cr, Sr, Sc, Y, Ba, a rare earth element, and a mixture thereof.
In order to provide a positive electrode active material for a non-aqueous electrolyte secondary battery having higher capacity than that of current LiCoO2, excellent in the thermal stability in a charged state, at a low cost, International Publication No. WO 2002/041419 (Patent Literature 2) discloses a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium complex oxide represented by the following compositional formula (1), the X-ray diffraction chart of the lithium complex oxide including diffraction peaks of a complex oxide of Li and W and/or a complex oxide of Li and Mo, in addition to the main diffraction peaks attributable to a hexagonal crystal structure:LiaNibCocMndMeO2  (1)
wherein M represents one or more selected from W and Mo, and 0.90≤a≤1.15, 0<b<0.99, 0<c≤0.5, 0<d≤0.5, 0<c+d≤0.9, 0.01≤e≤0.1, and b+c+d+e=1.
According to the description in Japanese Patent No. 2526093 (Patent Literature 3), use of a lithium-molybdenum oxide represented by Li2-xMoO3 as the positive electrode active material for lithium secondary batteries allows the voltage to increase, and thereby lithium secondary batteries having a high energy density can be obtained, wherein x satisfies 0<x≤1, and 2-x indicates that Li2MoO3 is first synthesized, and Li is then electrochemically removed from the Li2MoO3.